Group-directed communications are commonplace in enterprise and public safety communication systems. One mobile communication device (transmitter) transmits an audio signal to a given group (talkgroup) of receiving mobile communication devices (receivers). These receivers reproduce the audio signal through an amplified speaker. The reproduced audio signal is typically audible to people other than merely the user of the particular mobile device emitting the sound, notably in circumstances in which the amplification of each mobile device may be maximized due to the large amount of ambient noise.
In a number of situations, at least some of the mobile devices in the talkgroup are located near each other. This is particularly true in public safety incidences, in which personnel often respond to such incidences in a group, and this group (or a subset thereof) is located in the same local area for an extended period of time. If a transmitter and receiver are collocated, e.g., within a few meters of each other, the microphone on the transmitter may become acoustically coupled to the speaker on the receiver. In this case, the loop gain conditions may be such that acoustic coupling sets up a feedback loop that creates an undesirable acoustic signal often referred to as “howling,” owing to the piercing, shrill sound produced by the receiving radio speaker. Howling causes saturation in the communication channel and power amplifiers and speakers in the receivers which disrupts communication.
As above, such situations are especially prevalent in public safety incidents in which the transmitter and receiver(s) are often disposed within sufficient range and for a long enough time period to cause howling. While howling is irritating in normal situations, it is potentially disastrous during public safety incidents; notably if critical information is lost, misconstrued due to the feedback, or must be re-transmitted causing an unacceptable delay. It is therefore desirable to reduce, if not eliminate, instances of howling.
Some communication systems minimize howling by using signal processing techniques to detect and minimize the effect. Filters can be inserted into the signal path to decrease loop gain at frequencies where howling is likely to occur. Unfortunately, the desired signal is often undesirably distorted by the filters. In any event, these systems also only detect the phenomenon after it has started, which in emergency situations may be too late to have lost vital audio information or require repetition of such information, losing time that may be critical.
Accordingly, there remains a need for a method that reduces howling in an efficient and cost-effective manner.